The present invention relates generally to laser shock peening and, more particularly, relates to a system and method for monitoring the shock peening process through monitoring of a time-of-flight of the shock wave.
Laser shock peening or laser shock processing (LSP), is a process whereby a shockwave is impinged upon a surface of a part and produces a region of compressive residual stress in an outer layer of the part. It is well understood that the compressive residual stress in the outer layer of the processed part increases the service life of a processed part with respect to cyclic fatigue failure. Understandably, the ability of the part to withstand fatigue failure is dependent, in part, on the quality of the coupling of the shockwave with the part. That is, if the shockwave is not appropriately coupled at the surface of the part, the quality of the peen of the resultant peening process is detrimentally affected.
During the laser shock peening process, a laser generator creates a laser beam that is directed toward the part to be processed. Preferably, to improve the coupling of the energy of the laser beam with the part being processed, an absorption layer and a containment layer are positioned between the part and the laser generator. Generally, the absorption layer and the containment layer are positioned adjacent the part. The laser beam is allowed to pass through the containment layer, usually water, and impinge upon the absorption layer. The absorption layer is generally formed of a thin coating of tape, paint, ink, or foil and is commonly applied directly to the part being processed or maintained in very close proximity thereto. The containment layer is generally located adjacent the absorption layer between the absorption layer and the laser generator. The interaction of the laser beam with the absorption layer produces ablation of the absorption layer, which ultimately generates a shockwave that expands from the absorption layer/confinement layer interface. The containment layer ensures that a substantial portion of the initial shockwave is directed toward the part being processed and thereby enhances the coupling of the shockwave generated by the energy of the laser with the part.
Current practice of laser shock peening requires extensive destructive testing to ensure that parts being processed achieve a desired processing effect. That is, when several parts are to be processed, a select few of the total number of processed parts will be tested to failure to ensure the quality of the remainder of the parts. This type of destructive testing is often time consuming and expensive to implement and execute. Additionally, such testing provides no indication of a real-time, individual peen quality of the part being processed. The failure tested parts are fully processed prior to any testing. The processing of subsequent parts must then be suspended to allow time to failure test the part or continue with the potential of producing parts which do not satisfy quality criteria. Suspending the processing procedure and/or producing subsequent parts which do not satisfy quality criteria detrimentally affects overall process efficiency.
Other systems and processes attempt to improve the process efficiency through real-time sample part processing. That is, these systems process a much smaller part or coupon and determine the quality of the coupling through testing of the coupon. While these approaches improve the real-time aspect of quality control, they must also associate the quality of the test coupon coupling with coupling of an actual part being processed. Such testing can result in misleading data characterizations and associations when the data acquired is not associated with an actual part being processed. Still other systems non-destructively test the quality of the coupling through measurements of characteristics of a processed part such as surface hardness values and peen depth and shape data. Such approaches are incapable of accounting for real-time variations in the coupling quality and analyze only a very few of the many peen sites. Yet other systems monitor parameters and data acquired during processing of a part, destructively test a select group of parts, and compare the extensive data acquired during processing with the data of the destructively tested parts. Although this approach allows for a quality comparison of each part of a group of processed parts, the process still requires the destructive testing of a select group of parts to acquire the control quality data.
Therefore, it would be desirable to design a laser shock peening system and method capable of real-time non-destructive quality monitoring of the peening process.